Any naturally occurring inorganic salts of phosphoric acid, H₃(PO₄). All known phosphate minerals are orthophosphates since their anionic group is the insular tetrahedral unit (PO₄)³⁻. Mineral salts of arsenic acid, H₃(AsO₄), have a similar crystal chemistry. There are over 150 species of phosphate minerals, and their crystal chemistry is often very complicated. Phosphate mineral paragenesis can be divided into three categories: primary phosphates (crystallized directly from a melt or fluid), secondary phosphates (derived from the primary phosphates by hydrothermal activity), and rock phosphates (derived from the action of water upon buried bone material, skeletons of small organisms, and so forth).